Mixing separated ash from the flue gases with black liquor which is to be combusted in a recovery boiler is a well-known technique that has been practiced over many decades. Various installations for carrying out this kind of mixing have been constructed over the years. Such mixing apparatuses are referred to as mixer tanks and normally consist of a vessel equipped with a stirrer, a liquor inlet, a liquor outlet, ventilation pipes, an ash inlet and, in most cases, a heating device. The stirrers generally used have been slow-moving stirrers with arms that sweep along the periphery of the mixer tank and do not create sufficient turbulence to prevent fairly large particles from settling to the bottom zone. The lowermost arms of the stirrer are therefore designed as scraper tools which operate near the bottom and at that location effect a mechanical working of the sedimented material. The material is broken up into smaller particles which can then gradually be lifted upwards to a higher level in the tank and removed through the outlet.
To ensure that the liquor distribution in the furnace will function in a trouble-free manner, the injected liquor must not contain excessively large particles, since these can clog up the injection nozzles. For this reason, the liquor is usually screened through a fixed screen called a holed plate, which is placed in front of the outlet of the mixer tank.
One example of a mixer tank design is described in U.S. Pat. No. 5,405,502. The mixer tank is built into, or arranged on the outside of, the storage tank for black liquor. The apparatus is based on the abovementioned principle, but since the mixer tank is preferably arranged inside the storage tank for black liquor, the finish-mixed product is allowed to flow into the storage tank through an overflow, without passing through any screen or traveling via any pump.
A problem with the known apparatuses which effect slow stirring is that the mixture of ash and liquor is not as homogeneous as possible, and relatively coarse ash particles are not dissolved. These coarse ash particles can undesirably clog the screen plate and pumps. The ash which is supplied, and which for the most part consists of finely particulate material, has a tendency to lump together. These lumps may cause operating breakdowns. Furthermore, the lumps cannot be dissolved chemically by means of a long dwell time, since the liquor is concentrated and saturated with the same chemicals the ash comprises, namely various sodium salts, such as sodium carbonate and sodium sulphate. The ash may also sinters together in certain parts of the recovery boiler. When sintered coatings of this kind come loose, for example in association with soot-blowing, they fall downwards in the ash system. It is necessary for the large lumps to be broken up mechanically. Therefore, in most cases the lowermost arms of the stirrer have been designed as scraper tools which operate near the bottom and in that location effect a mechanical working of the sedimented material. Working the ash lumps breaks it up into smaller particles which can gradually be lifted upwards to a higher level in the tank and then removed through the holes in the screen plate. This results in undesirable wear and tear on the bottom scraper, which can be made worse if stones and metal scrap accompany the liquor or the ash into the tank.
There has therefore long been a need to be able to produce a mixer tank for black liquor and ash which overcomes the abovementioned problems.